Dye-sensitized solar cells are known as wet solar cells or Grätzel cells, which are characterized by being produced without silicon semiconductor and having an electrochemical cell structure using an electrolysis solution represented by an iodine solution. Specifically, dye-sensitized solar cells have a simple structure which includes: a porous semiconductor layer, such as titania layer formed by baking titanium dioxide powder on a transparent conductive glass plate (a transparent substrate having a transparent conductive film laminated on its surface) and making the powder adsorb a dye; a counter electrode made up of a conductive glass plate (a conductive substrate); and an electrolysis solution, such as an iodine solution, arranged between the porous semiconductor layer and the counter electrode.
Dye-sensitized solar cells have attracted considerable attention as low-cost solar cells because their materials are not expensive and they do not require any large-scale facilities.
However, under present conditions, the sunlight conversion efficiency in dye-sensitized solar cells is about 11%; and thus, further improvement in conversion efficiency is needed.
Considerations have been made at various angles to improve sunlight conversion efficiency. One proposal given is to raise the sunlight absorption efficiency by increasing the thickness of the porous semiconductor layer.
There is another proposal for dye-sensitized solar cells, though the proposal is applicable only to dye-sensitized solar cells that include a metal oxide semiconductor layer of ordinary thickness. Specifically, in order to raise the conversion efficiency by efficiently transferring electrons to the transparent conductive film, a comb-shaped conductive layer is formed in the metal oxide semiconductor layer (porous semiconductor layer) about 10 to 13 μm thick and the conductive layer and the transparent conductive layer on the substrate are short-circuited. Such a conductive layer is formed, by vacuum deposition or the like, on the metal oxide semiconductor layer formed to about 8 μm thickness which has its end portion scribed so that the transparent conductive layer is exposed and which has a comb-shaped mask fixed on its surface (refer to Japanese Patent Laid-Open No. 2003-197283).
There is also proposed a photo-electric conversion device which has a wire-mesh-like structure made of metal arranged in its photosensitive layer (layer including semiconductor) (refer to Japanese Patent Laid-Open No. 2005-285473).
However, increasing the thickness of the porous semiconductor layer also presents some problems. If the electron diffusion length exceeds the thickness dimension of the porous semiconductor layer, increasing the thickness of the porous semiconductor layer does not have the effect of raising the conversion efficiency; to the contrary, open-circuit voltage is decreased and the conversion efficiency is lowered.
Further, the process disclosed in Japanese Patent Laid-Open No. 2003-197283, in which a comb-shaped conductive layer is formed in the metal oxide semiconductor layer by vacuum deposition, has the effect of improving the conversion efficient, but will be complicated and costly. Moreover, whether or not the process is suitably applicable to cases where the thickness of the porous semiconductor layer exceeds 13 μm has not yet been proven.
The process disclosed in Japanese Patent Laid-Open No. 2005-285473, in which a wire-mesh-like structure made of metal is arranged in the photosensitive layer (layer including semiconductor), is also complicated, like the process disclosed in Japanese Patent Laid-Open No. 2003-197283. Besides, there is a limit to decreasing the thickness of the wire-mesh-like structure, and thus, the process may present the problem of short-circuit due to the contact of the wire-mesh-like structure with the counter electrode.
The present invention has been made in the light of the above problems. Accordingly, the object of the present invention is to provide a dye-sensitized solar cell which can be produced by a relatively easy and simple process and ensures high conversion efficiency even in cases where the thickness of the porous semiconductor layer is increased and to provide a process for producing the same.